Low-temperature heating systems based on enamelled steel sheet have long been known. In their case, electrical resistances in the form of heating lacquers, metal-containing pastes or metallic conductor lines are directly applied to the enamelling. This conventional enamelling, which in this case functions as an electrical insulator, has the disadvantage that its electrical volume resistance decreases with increasing temperature, so that the use of heating systems such as these is confined to a low temperature range of up to 150.degree. C.
GERMAN No. 3 536 268 describes a heating element for high inuse temperatures (&gt;150.degree. C.). The heating element disclosed is a composite system consisting of a steel plate to which an electrically insulating glass base layer, metallic conductor lines and a chemically resistant glass surface layer acting as a sealing layer are applied. This heating element is capable of withstanding temperatures of up to 400.degree. C. without any change in the resistance of the glass insulating layer. The electrically insulating glass layer used in this case consists of an alkali-free calcium-aluminium borosilicate (see also GERMAN No. 3,446,554).
The disadvantage of these heating elements is that the steel sheet has to be decarburized, degreased, pickled and nickel-plated so that the insulating glass layer adheres firmly to the steel sheet. The other heating elements described in the above-cited patent specification (where a steel sheet coated with a base enamel is used instead of a treated steel sheet) are attended by the disadvantage that the volume resistance of the insulating glass layer again decreases after a short time and after repeated heating and cooling of the element, so that the serviceability of the element is seriously affected or even destroyed.
Accordingly, the object of the present invention is to provide heating elements in which, on the one hand, the steel does not have to be pretreated and in which, on the other hand, the insulating glass layer on which the conductor lines are situated retains its volume resistance.